In a large rotating electrical machine, a large voltage of about several kilovolts to ten and several kilovolts is almost always applied between a coil portion which is a conductor and a slot insulation material which is an insulator. When partial discharge occurs at this portion, the insulation is damaged by ionized molecules, so that the lifetime of the rotating electrical machine is remarkably shortened. In particular, since the thickness of the insulation layer is reduced with the recent miniaturization of devices, and in turn the electric field intensity increases, the possibility of such partial discharge tends to increase. At the same time, the increase in generated (output) voltage and current with the increase in capacity can also be regarded as a factor of causing the above-described problem. Hence, in rotating electrical machines, particularly, large rotating electrical machines, a material capable of relaxing an electric field generated between a coil and an insulation material is extremely important for securing the reliability of an insulation system capable of reducing the generation of corona.
Conventionally, a method in which a conductive coating material is applied onto or impregnated into a topmost surface of an insulation layer is widely employed as a method for relaxing an electric field in a rotating electrical machine to which a high voltage is applied. However, this method has the following problems: the workability during production processes of devices is not necessary good; the influence of the vaporization of a solvent on the working environment is non-negligible; and the operation requires a long time. Moreover, this method has a problem in terms of the reproducibility of the conductivity (resistance value).
A method for solving these problems may be a method in which a conductive thin sheet material (for example, paper, film, tape, or the like) is wound or inserted. Particularly, in electrical and electronic devices to which a high voltage is applied, such as large rotating electrical machines, the temperatures of the devices increase greatly, and hence a highly heat-resistant material is required.
On the other hand, highly heat-resistant aramid papers, which serve as electric insulators or thin sheet structure materials, have been widely used as electric insulating materials in the above-described fields of rotating electrical machines (power generators and motors) and transformers and for electrical and electronic devices. The use of an aramid paper as a material for relaxing an electric field has been also examined by providing some conductivity to the aramid paper.
Patent Literatures 1 and 2 disclose papers which use an aramid fibrid with a carbon fiber or a metal fiber. However, each of these papers is not intended to be a material for relaxing an electric field described above, and hence is unsatisfactory in terms of conductivity and mechanical strength.
Moreover, Patent Literature 3 discloses a conductive aramid paper which is constituted of an aramid short fiber, an aramid fibrid, and a conductive filler such as a carbon fiber, and which has a low density and a high strength. However, in the method described in this patent, a sheet formed by wet paper making is not densified after the formation. Hence, although the conductive aramid paper has a high resin-impregnating ability, it is difficult to form the conductive aramid paper in a small thickness by this method, in other words, space-saving is hampered. In addition, since the surface is not smoothed, this conductive aramid paper has such a problem that the paper tends to be fuzzed when installed in an electrical device or on a conductor, for example.